In the conventional display of video data, the intensity of a scanning beam in a cathode ray tube is controlled as it scans along raster scan lines so as to create an image on the display tube. Typically, to control the color and intensity values displayed, a display memory is maintained having one memory address corresponding to each picture element or pixel on the display screen. A stream of ditigal data representative of the scan path of the cathode ray tube beam is fed to the display memory. The output of the memory at each address is fed to a ditigal to analog converter (DAC) which in turn provides an analog signal for controlling the CRT beam intensity at the pixel on the display corresponding to that address.
As a practical matter, the number of desired intensity values of the scanning beam is quite small. They may, for instance, include two values for "on" and "off", or may more generally include three primary colors and eight or sixteen intensity levels. This information may be stored as several bytes or less of data at each address of the display memory. When it is desired to simultaneously display information from several planes, however, it becomes impractical to store the display values separately for each plane. Instead, in order to control the scanning beam it is a common practice to maintain data representing these display values in a look up table (LUT). The display memory contains, at each storage location, a short data word The data words from corresponding points of all the planes are combined to form an address in the look up table, and a data word representing the desired display value is stored at that address By way of example, the display memory may consist of a 512.times.512.times.8 bit RAM, and the look up table may be a 128.times.8 bit RAM. Each of the addresses in the display memory may hold a single eight bit word which identifies an address in the look up table. Each of the 128 addresses in the look up table accesses a single eight bit word which represents the color and/or intensity value to be displayed.
Where the system is an engineering or a graphics display system, several planes P1, P2, . . . Pn of graphics must be stored in a manner for simultaneous display on the screen. In such a case, the intensity and color value V displayed at a point (x,y).sub.s on the screen will be a function of the corresponding points (x,y).sub.P1, (x,y).sub.P2 . . . (x,y).sub.Pn of the n planes. For example, if the stored planes all represent horizontal sections through a building, and the screen is to display a vertical view from above, the top plane could be displayed in its entirety at a first intensity or color, and the portions of the other planes be displayed only where they are visible through gaps in the top or overlying planes, each displayed at a progressively lesser intensity or different color. Similarly, for a perspective view, the intersection contours of non-parallel planes may be highlighted.
Such display presentations are conventionally implemented using a look up table, as shown in FIG. 1, below In such a construction, each point (x,y).sub.i in a plane P.sub.i stores part of an address in the look up table, and the output of the look up table is an illumination value V(P1, . . . Pn) which is a function of the n points (one in each plane) corresponding to the point (x,y).sub.s on the screen. With such a prior art look up table, if n planes are available for simultaneous viewing on the screen, the size of the memory required for the look up table varies as 2.sup.n. For n greater than approximately eight, a memory of suitable size has slow access times, in the range of 25 ns or more. Such access times impose limitations on the system design and performance.